usb: cdns3: include host-export,h for cdns3_host_init

The cdns3_host_init() function is declared in host-export.h
but host.c does not include it. Add the include to have
the declaration present (and remove the declaration of
cdns3_host_exit which is now static).

Fixes the following sparse warning:

drivers/usb/cdns3/host.c:58:5: warning: symbol 'cdns3_host_init' was not declared. Should it be static?

Signed-off-by: Ben Dooks <ben.dooks@codethink.co.uk>
Signed-off-by: Felipe Balbi <felipe.balbi@linux.intel.com>